jwo.utils.gifutils
Class GIFInputStream

java.lang.Object
  java.io.InputStream
      java.io.FilterInputStream
          jwo.utils.gifutils.BitInputStream
              jwo.utils.gifutils.GIFInputStream

All Implemented Interfaces:

Closeable

public class GIFInputStream
extends BitInputStream

GIFInputStream provides readXXX() methods useful for reading in the contents of a GIF file. Included are methods which handle LZW decompression of the image data.

Author:

Benjamin E. Norman. Very minor modifications, Jo Wood.

Field Summary

static int	MAXIMUM_CODE_LENGTH Maximum code size in bits for GIF LZW decompression
protected jwo.utils.gifutils.LZWStringTable	stringTable String table for LZW decompression

Fields inherited from class jwo.utils.gifutils.BitInputStream

cachedBits, numCachedBits

Fields inherited from class java.io.FilterInputStream

in

Constructor Summary

GIFInputStream(InputStream is)
Creates an input stream suitable for reading .gif files.

Method Summary

byte	readByte() Reads an 8 bit byte at the current bit boundary.
protected short	readCode(BitInputStream bis) Reads a code from the given BitInputStream using the code size from the instance LZW string table.
Color	readColor() Reads 3 bytes and interprets them as an RRGGBB hex color.
byte[]	readDataBlock() Reads data sub-blocks from the underlying input stream until a block terminator (a single zero byte) is encountered.
byte[]	readDataSubBlock() Reads a data sub-block from the underlying stream.
byte[]	readImageData(int length) Reads the compressed data for a table-based image and return the decompressed raster data.
int	readWord() Reads an unsigned LSB-first 16 byte word from the underlying stream.

Methods inherited from class jwo.utils.gifutils.BitInputStream

read, readBit, readBits

Methods inherited from class java.io.FilterInputStream

available, close, mark, markSupported, read, read, reset, skip

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

MAXIMUM_CODE_LENGTH

public static final int MAXIMUM_CODE_LENGTH

Maximum code size in bits for GIF LZW decompression

See Also:

Constant Field Values

stringTable

protected jwo.utils.gifutils.LZWStringTable stringTable

String table for LZW decompression

Constructor Detail

GIFInputStream

public GIFInputStream(InputStream is)

Creates an input stream suitable for reading .gif files.

Parameters:

is - Underlying input stream.

Method Detail

readByte

public byte readByte()
              throws IOException

Reads an 8 bit byte at the current bit boundary.

Returns:

Byte read from input stream.

Throws:

IOException - if 8 bits are not available.

readWord

public int readWord()
             throws IOException

Reads an unsigned LSB-first 16 byte word from the underlying stream.

Returns:

Word read from underlying input stream.

Throws:

IOException - if EOF is reached before 2 bytes are read.

readColor

public Color readColor()
                throws IOException

Reads 3 bytes and interprets them as an RRGGBB hex color.

Returns:

Colour read from input stream.

Throws:

IOException - if EOF is reached before 3 bytes are read.

readDataSubBlock

public byte[] readDataSubBlock()
                        throws IOException

Reads a data sub-block from the underlying stream. The first byte read is taken to be the sub-block's length and is not returned in the array.

Returns:

Data block.

Throws:

IOException - if a complete block is not read before EOF

readDataBlock

public byte[] readDataBlock()
                     throws IOException

Reads data sub-blocks from the underlying input stream until a block terminator (a single zero byte) is encountered. All bytes read except for the first byte of each sub-block are returned.

Returns:

Data block.

Throws:

IOException - if a complete block is not read before EOF

readCode

protected short readCode(BitInputStream bis)
                  throws IOException

Reads a code from the given BitInputStream using the code size from the instance LZW string table. Therefore, this should only be called after (usually by) readImageData(), which instantiates the string table.

Returns:

Code.

Throws:

IOException - if a code is not read before EOF

readImageData

public byte[] readImageData(int length)
                     throws IOException

Reads the compressed data for a table-based image and return the decompressed raster data. Each byte in the return array will correspond to one pixel, regardless of image bit depth. The root code size is read as a single byte before the image data sub-blocks. The length of the uncompressed data is required so that the array may be created; this should be simply the image width times the image height.

Parameters:

length - Amount of data to read in bytes.

Returns:

Image data.

Throws:

IOException - if EOF is reached prematurely or the LZW decoding fails